When the stem holding the seedhead is completely brown, try this test.

Take a big bowl, or a paper bag out to the garden. Bend a seedstem down so the seedhead is inside the bag or bowl. Shake it. If the seeds fall off, they are ripe and ready to harvest. Usually, the seedheads will open up, and the seed carriers will protrude from the stems, instead of laying along the stem, as they are nearing ripeness. Mine aren't ready yet.

Most Festuca will produce viable seed. Parentage, however, is not guaranteed. Any group of seedlings from one parent plant will exhibit a variety of characteristics. Some may be like the parent, and some may not.